Try to improve error messages when creating Variables

[ZedThree]

Error messages are currently reported directly from the netcdf-C error string, which doesn't include information about the specific object. This can make debugging awkward or painful. This PR simply includes the variable and group names in error messages arising from Variable.__init__.

Minimal example:

import netCDF4

with netCDF4.Dataset("badname.nc", "w", format="NETCDF4") as f:
    time = f.createDimension("time", None)
    g = f.createVariable("badname ", "i4", ("time",))

Error without this PR:

RuntimeError: NetCDF: Name contains illegal characters

With this PR:

RuntimeError: NetCDF: Name contains illegal characters: (variable 'badname ', group '/')

Exact format could be tweaked and used in more places.

[ZedThree]

Note that unfortunately this likely can't be solved in the netCDF-C library itself because the error strings are const char*, so I don't think there's a good way to include dynamic information in the error message.

[DennisHeimbigner]

The alternative is to log a more complete message before returning the error.

[jswhit]

looks good - I can merge once you update to the current master and include a Changelog entry